Traditionally, radio telecommunication systems have been designed almost exclusively for voice or for packet data. The delay or latency requirements, the bursty nature of communications and the asymmetry of the traffic in both cases are so different that separate designs are often proposed for the two different types of transmissions. Generally, voice allows only short delays, has a roughly symmetrical load in uplink and downlink and is not bursty. On the other hand, packet data transmissions can be very asymmetrical (e.g. a browser communicating with websites over the Internet), is often delay tolerant and is often bursty in nature. There have been several attempts to design systems to provide both data and voice in the same system. One such proposal is the ETSI General Packet Radio Service (GPRS) which is an overlay network on the circuit switched GSM system. A GPRS architecture proposed by ETSI in Technical Specification 3.6 is shown in FIG. 1. Shown mainly on the left of the diagram is a conventional GSM mobile telephone system for full duplex voice communications comprising a Mobile Switching Centre (MSC) a Base Station System (BSS) usually including a Base Station Controller (BSC) and a Base Transceiver Station (BTS), and a mobile terminal (MT) and a Home Location Register (HLR). Packet data services are limited to the Short Message Service (SMS) which is dealt with by an SMS Gateway Mobile Switching Centre (SMS-GMSC) and a Short Message Service Centre (SM-SC). Fax is dealt with as in an ordinary telephone system, e.g. via suitable modems and an Interworking Function (IWF) fax data is transmitted via circuit switching. Hence, conventional mobile telecommunications systems generally use what may be described as circuit switched data transmissions. GPRS adds two new nodes to such a system, namely the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support node (GGSN), both of which may be seen as routers. The SGSN contains the identity of MT in its routing tables which are inserted when the MT registers with the network. The GGSN is connected to other data carrying networks, for example a Packet Data network (PDN), for the receipt and transmission of packets of data. As the GPRS system is in parallel to the GSM system information about change of location of the MT is also sent to the SGSN/GGSN.
The above hybrid system may be adapted to a Third Generation Mobile Telephone system such as the UMTS system as shown schematically in FIG. 2. Further details of such an implementation may be found in the book by Ojanperå and Prasad, “Wideband CDMA for Third Generation Mobile Communications”, Artech House Publishers, 1998. Basically, the Radio Access Network (RAN) provides the network-side equipment for communicating with the MT. A GPRS SGSN and a UMTS MSC are provided in parallel between the RAN and the relevant network, i.e. or a PDN or a Public Service Telephone Network (PSTN), respectively.
GPRS provides a connectionless support for data transmission. However, in order to use the scarce resources on the radio air interface between the BTS and the MT, a circuit switched radio resource allocation is used. Thus, although the networks attached to the GGSN may operate in a completely connectionless way, the transmission of the data packets across the air interface makes use of conventional timeslot and frame management. Accordingly, at some position in the GPRS network a packet handler is required which prepares the packets for transmission in frames across the air interface and receives the frames from the air interface and prepares them for transmission to the data network. This unit may be called a Packet Control Unit (PCU) and may be placed at several alternative positions, e.g. in the Base Transceiver Station (BTS), in the Base Station Controller (BSC) or between the BSC and the SGSN. Generally, the PCU may be assigned to some part of the BSS—the base station system. Typically frame relay will be used between the PCU and the SGSN.
One particular advantage of GPRS is that several MT's can camp onto a single timeslot. The data blocks destined for a particular MT are identified by a specific Temporary Flow Identity (TFI). Hence, each MT which shares a timeslot with others decodes each block to determine if the block contains its TFI. Once a data transmission is completed the same TFI may be used for another transmission either with the same MT or with another MT. The TFI mechanism allows some optimization of usage of the radio resources. Whereas full duplex circuit switched transmission with packet data may include silences, the shared timeslots may be optimally filled with data for different MT's. This provides some optimization of the radio resource despite the bursty nature of data traffic. At the same time this data network is operated in parallel with voice communications, the basic protocols of both systems being the same.
The protocol stacks used by GPRS are shown in FIGS. 3 and 4. The GPRS protocols make a difference between transmission and signaling. The transmission protocols provide data transfer but also associated control information such as flow control, error detection, error correction and error recovery. The signaling plane provides control and signaling for attach and detach form the GPRS system, for controlling the routing path during user mobility, for controlling the assignment of network resources and providing supplementary services.
The setting up of circuit switched calls across the air interface in a GPRS network is shown in message flows in FIGS. 5 and 6. In FIG. 5 a data request is initiated by a mobile terminal (MT) using an access control channel, e.g. a Random Access Channel RACH. When a MT has some data to send it makes an Uplink Radio Connection Establishment Request specifying how much data is to be sent. The RAN replies with a confirmation message that the uplink radio link is provided and gives details of when and how the MT is to transmit, e.g. which timeslot and how much of the timeslot can be used. Then the data is transmitted by the MT on a traffic channel and the RAN disconnects the radio link after all data has been transmitted successfully. The data received by the RAN is forwarded to the SGSN and from there to the GGSN which removes any headers used for transporting the data up to this point and transfers the data to the relevant PDN, e.g. via the Internet to a remote server. As some time later the answer to the data arrives from the remote site, e.g. a service provider's server on the Internet. On receipt of this answer a downlink radio connection is set up by the RAN via a control channel and the answer data transferred via a traffic channel. After transfer the radio connection is released once again.
FIG. 6 shows a similar message scheme when the initiating message is downlink. Again, the downlink and uplink transfers are not coupled so that the downlink radio connection is released at the end of the downlink transmission and before the answering uplink transmission.
The above message procedures have a disadvantage. It is necessary to set up and tear down radio connections between each uplink and downlink data transfer. This causes delays as the network must wait until the radio resource is ready before the data can be transmitted. Any delays can be annoying to the MT user who expects from his/her mobile browser the same performance as with landline systems. From a system point of view the delay on the answering uplink transmission may not be so severe as the MT has its data usually stored on a suitable device, e.g. a lap- or palmtop, where storage space is probably not a limitation and the amount of data is usually small, anyway. On the other hand, the data coming from the Internet on the downlink in answer to a request from an MT is often of large volume. This means that a large buffering capacity must be provided in the GPRS system to keep the received data until the radio resources are ready. The provision of buffering capacity is described in EP-A-332 818. Due to the high asymmetry expected with browser traffic (estimates are at least 10 to 1), the level of buffering capacity on the downlink can be high which is costly to implement.
It is an object of the present invention to provide a data carrying cellular mobile radio telecommunications system and a method of operating the same which reduces delays, in particular on the downlink.